In many cryogenic applications components, e.g. superconducting coils for magnetic resonance imaging (MRI), superconducting transformers, generators, electronics, are cooled by keeping them in contact with a volume of liquefied gas (e.g. Helium, Neon, Nitrogen, Argon, Methane), the whole cryogenic assembly being known as a cryostat. In order to operate a superconducting magnet, it must be kept at a temperature below its superconducting transition temperature. For conventional low temperature superconductors, the transition temperature is in the region of 10K, and typically the magnet is cooled in a container or vessel comprising a bath of liquid helium, commonly called a helium vessel, at 4.2K. For simplicity, reference shall now be made to helium, but this does not preclude the use of other gases. Services need to be run from the external environment at room temperature into the helium vessel, for monitoring purposes and to energize the magnet. Any dissipation in the components or heat getting into the system causes helium boil-off. To account for such losses, replenishment is required. This service operation is considered as problematic by many users and great efforts have been made over the years to introduce refrigerators that either reduce the rate of boil-off, or recondense any lost liquid back into the bath.
In many cryostats the liquid gas boils away slowly as a result of heat entering the system. A suitable means must be available for the gas to exit from the cryostat, but it is one function of the cryostat to reduce this boiling to as low a value as practical since gases such as helium are expensive commodities. In other cryostats, a refrigerator is fitted, which recondenses the evaporated gases so that there is no overall loss of helium. In these cryostats, the heat load must be kept low enough that the refrigerator can perform the recondensation.
A cryostat must provide access to the vessel containing the liquified helium for the initial cooling of the magnet to its low operating temperature, and for periodic refilling of systems where there is a loss of helium. Furthermore, the cryostat must provide access to the helium vessel to measure the level of the liquified helium, and provide sufficient access whereby to enable operation and maintenance of the magnet. The magnet typically comprises one or more superconducting electromagnetic coils in series connection with a superconducting switch so that the field can be trapped in the magnet. Heat must be supplied to the superconducting switch to heat it above its superconducting transition temperature in order to “open” it. Electric current must be supplied to the magnet in order to energize it.
Electric current for the magnet is conveniently supplied through a removable current lead which is inserted through the access neck and provides electrical contact between an electrical terminal of a magnet at 4.2K and external cables at room temperature which connect to a power supply. Alternatively, a set of fixed current leads have been used which are permanently installed in the access neck so that the neck does not have to be opened to atmosphere in order to insert a removable current lead. Opening the neck tube to the atmosphere is to be avoided as there is the possibility of air entering the neck and helium vessel. This is to be avoided since air at temperatures below 0° C. (at normal atmospheric pressure) will include ice from water and, if present in the necks, would tend to collect at the bottom of the neck and either block the neck or prevent access to the magnet electrical terminal. Fixed current leads add to the heat load on the helium vessel.
Once the magnet has been energized, should an emergency situation arise which requires that the magnetic field be discharged rapidly, the magnet must be “quenched”. This involves heating a section of the magnet above its critical temperature so that it becomes resistive. The heat generated in this resistive section heats the adjacent parts of the magnet and causes them to become resistive. In this way the whole magnet rapidly becomes resistive, and the magnetic field is rapidly reduced to a negligible amount. The energy stored in the magnet is released into the liquid helium with the subsequent evolution of a large quantity of gas. The gas flow in this process can be high, and the access neck must provide a path for the gas to escape from the helium vessel without causing an excessive pressure in the helium vessel. The above, and other services, are provided through the service neck.
An example of prior art, comprising a conventional access neck is shown in FIG. 1: a tube 10 connects the vacuum vessel 12 at room temperature with a helium vessel 14 at a superconducting temperature, e.g., 4.2K. A vacuum exists external to the tube 10; helium gas is present inside. A guide tube 16 provides a guide for a removable current lead (not shown) so that it engages on a magnet connector 18. The guide tube is fitted with one or more radiation baffles 20 to reduce the amount of thermal radiation passing from room temperature to the helium vessel. Thermal connections 24 external to the tube 10 are connected to a cooling device (not shown) to intercept conducted heat.
There are several disadvantages of such an access neck configuration. Firstly the neck must be opened in order to insert the removable current lead, with the possibility of admitting air to the helium vessel. Secondly, there is no means of providing a controlled de-energization of the magnet except by fitting the removable current lead, which means that a trained service engineer is required. Thirdly, the back pressure during a quench process is high because of the use of multiple radiation baffles. Furthermore the heat load at the magnet connector is typically high during energization of the magnet, leading to high helium loss. Additionally the thermal connections 24 connect only to the outside of the service neck tube 10 and as such are not ideal because of non-optimal thermal contact with the gas in the neck tube.
A further prior example is shown in FIG. 2. This alternative access neck contains fixed current leads 30, comprising tubes of a moderate thermal conductivity material such as brass so as to conduct little heat into the system whilst having convenient dimensions for carrying electric current. This design is well known to those skilled in the art. The leads are mechanically secured by at least one collar 24 which also provides a means of conducting heat from the tubes to a heat sink (not shown). Item 32 is a means of electrically isolating the one or more collars 24 from at least one of the conductor tubes 30 whilst providing good thermal contact between them. Fixed electrical contacts 34 provide a means of electrical connection for electrical cabling 36, 38 to the magnet which have low electrical resistance.
Some of the disadvantages of this access neck are that the back pressures developed during a quench process can be high; this is because the gas must be vented primarily up the fixed leads in order to ensure that they are adequately cooled during magnet energization. The cooling of the gas column is not particularly efficient because the boil-off gas passes primarily up the two fixed leads. The diameter of the leads cannot be made large because other service operations and fittings must also be provided through the neck and if the neck diameter is increased the heat load increases.
Furthermore, there are three paths for gas going up the neck, two inside the current leads and one through the surrounding space inside the neck wall. In order to achieve optimum cooling of the current leads during a magnet ramp, gases should pass through the leads only and not through the third path. However, to achieve minimum helium losses during normal standby operation, with no current flowing in the leads, it is preferable to have some of the boil-off gas going through the third path, cooling both the neck as well as the leads. These conflicting requirements lead to a higher boil-off of gases then is preferred. Balancing the three parallel gas streams in a neck assembly requires precise knowledge of the gas impedances which is hard to predict and even harder to control in taking manufacturing tolerances into account.